Sound generation in metal flue organ pipes: On the influence of asymmetric flue exit geometry and vibrations of the upper labium with particular emphasis on the early response.

Doktorsavhandling, 2006

Sound generation in flue organ pipes is caused by the non-linear interaction between an instable jet and a tube resonator. Our understanding of this very complex physical process is still incomplete, and there is not yet a model that can explain all details of the richness in pipe sound which can be achieved by a trained organ pipe voicer. The perceived tonal quality is substantially due to the initial transient.
Central to the phenomena during the initial stages of sound generation is the motion of the jet in the pipe mouth. The first part of this thesis addresses the influence of asymmetric flue exit geometry details on the initial jet formation. The issue is investigated by means of computational fluid dynamics. A focus is on aspects of flow-separation and the interaction between the free jet and solid surfaces close to the flue exit. Jet velocity profile and direction are the result of a complex mixture of the free flow from the foot and flow-separation from the labium and languid edges. Lowering the languid, changing the languid edge and/or the angle of the lower labium are efficient means to redirect the jet. It is shown here that this also affects jet width; either a contraction or an increase in width can be the result, depending on the type of adjustment. A significant aspect in the jet formation from the flue exit of a metal flue organ pipe is the interaction between the free jet and languid front. This leads to a lateral disturbance of the jet tip. Further, simulations presented here show that due to the Coanda effect, the flow is attracted towards the languid for sufficiently small angles between languid front and free jet. This suggests that in a compact model of the sound generation process, the usual assumption of a jet emerging from a slit in a plane surface may not be sufficient to describe the flow's dynamic behaviour.
The second part of the thesis considers the influence of yielding pipe walls on the sound generation process with a focus on the interaction between upper labium and sound field. To investigate this, a metal flue organ pipe is artificially excited with a shaker at the labium. Responses of the pipe body measured with this loading are then compared to data obtained for the pipe when blown. Results suggest that the pressure exerted on the labium is not the dominating cause for vibrations of the pipe body. However, measurement data and numerical simulations show the labium is a very sensitive element, and results from the numerical studies help demonstrate the strong dependence of coupling between sound field and pipe walls when dominating frequencies in the pressure field about the labium are close to the natural frequencies of the structure.